1. Field of the Invention
The present invention generally relates to an optical registration system for an exposure apparatus, and more particularly, to an optical registration system for aligning printed circuit boards with two artworks, while compensating for any variations in glass, artworks, or boards to be exposed, in a double-sided exposure apparatus, and a data processing method therefor.
2. Description of the Background Art
During production of printed circuit boards, a wiring pattern is printed on a blank copper-clad circuit board or panel. The blank panel is moved into an exposure machine. The pattern is carried on a master known as an artwork, which is formed by resin or by a glass substrate with a thin film coating, such as mylar film. The circuit pattern appears in opaque lines on the otherwise transparent film or substrate. The fragile, heat-sensitive artwork or phototool is typically supported on a glass plate or artwork holder. The exposure machine applies ultraviolet (UV) light through the artwork to imprint the circuit pattern on a light-sensitive coating on the board. The copper underlying the coating is then chemically stripped off.
Generally, such exposure machines with optical registration only can print on one side of the board at a time. However, it is desirable to print on both sides of the board at the same time to speed up production and increase accuracy. One such exposure machine that prints on both sides of the board is disclosed in U.S. Pat. No. 4,764,791 (Omata et al.).
Double-sided boards usually have component mounting holes extending through the board. Before double-sided exposure, each upper and lower artwork and the board must be aligned correctly with respect to each other. In Omata et al., alignment is performed before the board enters the exposure chamber of the exposure machine. This alignment can then be checked by a camera. If the alignment is improper, the board is rejected and realigned or otherwise removed from the machine. Since alignment takes place outside the exposure chamber, misalignment is likely due to positional shifts caused by moving the board from the alignment station to the exposure position. Moreover, when the artworks are brought into contact, inaccuracies can occur from variations in the horizontal planar surfaces of each material, e.g., warpage and shape variations of the boards and the glass plates which are part of each artwork holding structure and/or expansion of the glass or artworks due to heat during the course of a production run.
Alignment inside the exposure chamber is described in Miyake U.S. Pat. No. 4,842,412. However, the alignment is only for a single side of the circuit board at a time.
A very high degree of precision in processing of printed circuit boards is now in demand. Surface-mount integrated circuit technology requires a finer pitch package, e.g., as little as 4 mils or less between leads. Thus, there is a need for a double-sided exposure machine that accurately aligns both artworks with each side of the circuit board and can compensate for sag of glass supporting an artwork and board variations.
There is a need for an accurate, fast double-sided exposure system which can use a machine vision system to achieve high accuracy.
There is also a need for a double-sided exposure system capable of using modern software technology to provide a user interface which is simple and easy to use.
Another need in the art is for a programmable system in which principal processing steps are written in a high-level computer programming language, enabling user modification without assembly language programming required by prior art systems.